1. Field of the Invention
The present invention relates to a process for producing plasticizer esters which are used well in various plastics including a vinyl chloride resin. More specifically, the present invention relates to a process for producing a high-quality plasticizer ester required for plasticized soft vinyl chloride which is widely used in an equipment for medical care, a wire cable, an insulating material and the like from an organic acid or its anhydride and an alcohol.
2. Description of the Prior Art
Plasticized soft vinyl chloride which is widely used in the medical field, the electric field, the electronic field and the like requires a high-quality plasticizer ester. For example, a plasticizer free from toxic coloring impurities, namely, a plasticizer ester having a low degree of coloring is required in the medical field; and a plasticizer ester having a high volume resistivity is required in the electric field.
The coloring impurities in the plasticizer are divided into those derived from starting materials and those formed during the production step. With respect to impurities in starting materials, for example, aldehydes in an alcohol, unsaturated compounds, sulfur compounds and the like, or materials from an acid source, for example, quinone substances in phthalic anhydride, a purification technique for removing these impurities has been developed, and impurities of starting materials have been, therefore, considerably improved. However, esters are different at times in a degree of coloring thereof depending on esterification reaction conditions even starting from the same starting materials.
As a method for forming a plasticizer ester having a low degree of discoloring, the following methods are known.
(1) A method in which when monoesterification is conducted by the reaction of an organic acid and an alcohol under normal pressure, a predetermined amount of water is added to a reaction vessel to form a state where a surface of a reaction solution is coated with a water vapor, whereby oxygen in air is prevented from being brought into contact with the reaction solution to improve the hue (Japanese Patent Laid-Open No. 113,814/1976), (2) a method for producing an plasticizer ester having quite a low degree of coloring by causing a bisulfite to exist in a reaction system (Japanese Patent Laid-Open No. 41,742/1976), (3) a method for producing a plasticizer ester having a low degree of coloring in which oxalic acid is added when subjecting an organic acid or its anhydride and an alcohol to an esterification reaction in the presence of an organic metal compound catalyst (British Patent No. 1,565,663), (4) a method in which a carboxylic acid ester is treated with an oxidizing agent such as hydrogen peroxide, sodium chlorite or the like and then with an alkaline aqueous solution, and the thus-treated product is washed with water, and further treated with a reducing agent such as sodium borohydride, hypophosphorous acid or the like for decoloration (Japanese Patent Laid-Open No. 22,618/1980), (5) a method in which a carboxylic acid is reacted with an alcohol, and the resulting product is purified by passing ozonization air therethrough under specific conditions (Japanese Patent Laid-Open No. 39,296/1981), and (6) a method in which an ester is continuously decolored in the presence of a Pd catalyst under hydrogenation conditions (Japanese Patent Laid-Open No. 110,650/1981).
Meanwhile, as a method for producing a plasticizer ester having a high volume resistivity, the following methods are known. (7) A method in which an esterification reaction product is heat-treated with a solid alkali such as sodium carbonate in the absence of water, and a reaction product is adsorbed with an adsorbent such as activated china clay (Japanese Patent Laid-Open Nos. 76,517/1979, 27,518/1979 and 27,519/1979), (8) a method in which a crude ester obtained by an esterification reaction is treated with oxidized magnesium silicate (British Patent No. 1,096,917 and the like), (9) a method in which the treatment is conducted with an absorbent such as magnesium oxide, activated carbon or the like (Japanese Patent Laid-Open No. 267,341/1987), and (10) a method in which an organic metal catalyst in an esterification reaction solution is hydrolyzed, neutralized with an alkali, and washed with water, an excess alcohol is recovered, and the resulting ester is treated with an adsorbent such as activated carbon or activated china clay (Japanese Patent Laid-Open No. 130,937/1980 and the like).